The invention relates generally to semiconductor device fabrication and, in particular, to heterojunction bipolar transistors, as well as fabrication methods and design structures for a heterojunction bipolar transistor.
Bipolar junction transistors are three-terminal electronic devices that include three semiconductor regions defining an emitter, an intrinsic base, and a collector. Heterojunction bipolar transistors are a variant of bipolar junction transistors in which at least two of the semiconductor regions are comprised of different materials. An NPN heterojunction bipolar transistor includes two regions of n-type semiconductor material constituting the emitter and collector, and a region of p-type semiconductor material sandwiched between the two regions of n-type semiconductor material to constitute the intrinsic base. A PNP heterojunction bipolar transistor includes two regions of p-type semiconductor material constituting the emitter and collector, and a region of n-type semiconductor material sandwiched between the two regions of p-type semiconductor material to constitute the base. The differing conductivity types of the emitter, base, and collector define a pair of p-n junctions, namely a collector-base junction and an emitter-base junction, across which the conductivity type changes. A voltage applied across the emitter-base junction controls the movement of charge carriers that produces charge flow between the collector and emitter.
Heterojunction bipolar transistors may be found, among other end uses, in high-frequency and high-power applications. In particular, heterojunction bipolar transistors may be used in radiofrequency integrated circuits, which are found in wireless communications systems, power amplifiers in cellular telephones, and other varieties of high-speed end uses. To take advantage of the positive characteristics of both transistor types, heterojunction bipolar transistors and CMOS field effect transistors may be combined in a single chip to form a BiCMOS integrated circuit.
Improved device structures, fabrication methods, and design structures are needed for enhancing the performance of heterojunction bipolar transistors.